1. Field of the Invention
The present invention relates to a method and related apparatus for performing status report procedure in a wireless communication system, and more particularly, to a method and apparatus for performing status report procedure in a wireless communication system when flexible PDU size is configured, so as to reduce system overhead.
2. Description of the Prior Art
The third generation (3G) mobile telecommunication system has adopted a Wideband Code Division Multiple Access (WCDMA) wireless air interface access method for a cellular network. WCDMA provides high frequency spectrum utilization, universal coverage, and high quality, high-speed multimedia data transmission. The WCDMA method also meets all kinds of QoS requirements simultaneously, providing diverse, flexible, two-way transmission services and better communication quality to reduce transmission interruption rates.
The access stratum of the 3G mobile telecommunication system comprises a radio resource control (RRC), radio link control (RLC), media access control (MAC), packet data convergence protocol (PDCP), broadcast/multicast control (BMC) and other sub-layers of different functions. The operations of the above-mentioned sub-layers are well known for those skilled in the art, and will not be further mentioned. A primary function of the RLC layer is providing different transmission quality processing, performing segmentation, reassembly, concatenation, padding, retransmission, sequence check, and duplication detection on received data or control instructions based on different transmission quality requirements. The MAC layer can match packets received from different logic channels of the RLC layer to common, shared, or dedicated transport channels according to radio resource allocation commands of the RRC layer, for performing channel mapping, multiplexing, transport format selection, or random access control.
In the RLC layer, the purpose of “padding” is to make lengths of all Protocol Data Units (PDUs) outputted from an RLC entity to be the same. That is, after the RLC entity receives a Service Data Unit (SDU) from the upper layer, if the length of the SDU is smaller than a maximum PDU payload size, the RLC entity will pad out the SDU with meaningless data, so as to form a PDU in conformation with a predefined length. Otherwise, if the length of the SDU is greater than the maximum PDU payload size, the RLC entity will segment the SDU with the maximum PDU payload size. After the segmentation, if the length of the last segment is smaller than the maximum PDU payload size, the RLC entity will pad out the last segment with meaningless data, to form a PDU in conformation with the predefined length.
In the prior art, “padding” can make the lengths of the RLC PDUs to be identical to each other. However, the prior art “padding” decreases bandwidth utility rate and data processing efficiency, especially for high data rate applications, such as High Speed Downlink Package Access (HSDPA) and High Speed Uplink Package Access (HSUPA) in the 3G mobile telecommunication system. In such a situation, a scheme of flexible RLC PDU size is proposed to eliminate padding, to enhance bandwidth utility rate and data processing efficiency, and to improve uplink and downlink (UL/DL) transmission rate.
For example, please refer to FIG. 1, which illustrates a schematic diagram of an application of the flexible RLC PDU size according to the prior art. In FIG. 1, SDU_1 and SDU_2 represent SDUs from the upper layer, PDU_1˜PDU_3 represent RLC PDUs, oblique-line parts in front of PDU_1˜PDU_3 represent PDU headers, and MPZ represents the maximum PDU payload size. As shown in FIG. 1, the total length of SDU_1 and SDU_2 is greater than two times MPZ, but smaller than three times MPZ. Therefore, the prior art uses a segmentation method to carry SDU_1 and SDU_2 with PDU_1˜PDU_3. The segmentation method is: if the length of one or concatenated SDUs is greater than one or multiple times of MPZ, the RLC entity will segment the SDU with a unit of MPZ until a last segment or SDU smaller than MPZ is left, and carry the last segment or SDU with a flexible-size RLC PDU. In other words, the lengths of PDU_1 and PDU_2 are equal to the maximum PDU size, while the length of PDU_3 is smaller than the maximum PDU size.
To support the flexible RLC PDU size feature, a segmentation function is added in the MAC layer. According to what is specified in related RLC and MAC specifications, a disadvantage, which makes radio resource usage inefficient, can be found as follows.
Firstly, a concept of Piggybacked STATUS PDU is to fully utilize the radio resources by using the padding part in an RLC AMD (Acknowledgement Mode Data) PDU. However, an AMD PDU with flexible RLC PDU size has no padding. In this case, there is no piggybacked STATUS PDU used when the flexible RLC PDU size is configured. However, lossless reconfiguration from fixed to flexible RLC PDU size is introduced recently. If an AMD PDU with fixed RLC PDU size including a piggybacked STATUS PDU is ongoing during reconfiguration, the receiver can receive this AMD PDU after changing to the flexible RLC PDU size and discard it if the receiver interprets it as an invalid PDU format.
Secondly, from the function point of view in the RLC layer, a STATUS PDU piggybacked or not does not have any impact. However, when RLC/MAC is considered together, such as in a Long Term Evolution (LTE) system, overhead occurs if the STATUS PDU is not piggybacked. For example, please refer to FIG. 2, which illustrates a schematic diagram of transmission of an RLC AMD PDU and an RLC STATUS PDU according to the prior art. After receiving an RLC SDU and SUFIs (Super Fields) from the upper layer, the RLC layer adds corresponding RLC headers to form the RLC AMD PDU and the RLC STATUS PDU. Then, the MAC-d (dedicated) layer forms MAC-d PDUs, or MAC-ehs SDUs, for the MAC-ehs layer, so as to form MAC-ehs PDU after adding a MAC-ehs header. As shown in FIG. 2, since no piggybacked STATUS PDU can be used due to flexible RLC PDU size, the STATUS PDU cannot be piggybacked by the RLC AMD PDU. As a result, two sets of parameters, including LCH-ID, L, TSN, SI, and F, in the MAC-ehs header are required, and thus overhead occurs. Note that, meanings of the parameters LCH-ID, L, TSN, SI, and F can be found in related specifications, and will not be narrated in detail for clarity.
Therefore, when flexible RLC PDU size is configured, an AMD PDU with flexible RLC PDU size has no padding. In such a situation, the RLC STATUS PDU cannot be piggybacked by the AMD PDU, and overhead occurs.